Distortion

Short story here - the Ai-P 45mm f/2.8 is basically free of distortion (0.2%).

The chart above has a real-world size of about 120x80cm.

Vignetting

The Ai-P 45mm f/2.8 is a full format lens thus enjoying a sweet spot advantage
on APS-C DSLRs. Nonetheless it still produces a relatively pronounced max.
vignetting of 0.74 EV at wide-open aperture. However, from f/4 the issue is
negligible.

MTF (resolution)

The Nikkor performed quite impressively in the MTF lab with a very even
center to edge resolution at all focal lengths. Compared to ordinary
normal lenses the max. resolution is lower at comparable large aperture
settings whereas the border performance is about as good.
At and beyond f/5.6 the quality is exceptional and about as good as it gets on
the D200.

Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Imatest Explanations

Chromatic Aberrations (CAs)

Chromatic aberrations (color shadows at harsh contrast transitions) are
negligible at f/2.8 but increase constantly along the tested aperture
range. However, even at f/11 the problem remains moderate with a
maximum below 1 pixel on the average at the image borders.

Verdict

The Nikkor Ai-P 45mm f/2.8 may be a cute lens but it is also a very serious
performer with very high resolution, marginal distortions, moderate CAs
and quite well-controlled vignetting. The build quality is very good.
The problem may be that today it represents an answer to a question which
was asked long ago. f/2.8 is not exactly fast for a normal lens so the
more common f/1.4 and f/1.8 variants tend to make more sense ... at lesser
costs. So at the end of the day the meaning of the lens lies in its extremely
small form factor. But damn, it is so cute ...